Light-emitting element, light-emitting device, electronic device, and lighting device

ABSTRACT

A light-emitting element emitting phosphorescence and having high emission efficiency, in which a property of injecting holes to a light-emitting layer is increased, is provided. The light-emitting layer of the light-emitting element includes a first organic compound represented by the following general formula (G1) and a second organic compound which is a phosphorescent compound. The difference between the HOMO level of the first organic compound and the HOMO level of the second organic compound is lower than or equal to 0.3 eV.

TECHNICAL FIELD

One embodiment of the present invention relates to a light-emittingelement in which an organic compound capable of providing light emissionby application of an electric field is provided between a pair ofelectrodes, and also relates to a light-emitting device, an electronicdevice, and a lighting device including such a light-emitting element.

BACKGROUND ART

A light-emitting element using an organic compound as a luminous body,which has features such as thinness, lightness, high-speed response, andDC drive at low voltage, is expected to be applied to a next-generationflat panel display. In particular, a display device in whichlight-emitting elements are arranged in matrix is considered to haveadvantages in a wide viewing angle and excellent visibility over aconventional liquid crystal display device.

A light-emitting element is said to have the following light emissionmechanism: when voltage is applied between a pair of electrodes with anEL layer including a light-emitting substance provided therebetween,electrons injected from the cathode and holes injected from the anodeare excited in a light emission center of the EL layer, and energy isreleased and light is emitted when the excited state returns to a groundstate. The excited states generated in the case of using an organiccompound as a light-emitting substance are a singlet excited state and atriplet excited state. Luminescence from the singlet excited state (S1)is referred to as fluorescence, and luminescence from the tripletexcited state (T1) is referred to as phosphorescence. The statisticalgeneration ratio of the excited states in the light-emitting element isconsidered to be S₁: T₁=1:3.

Therefore, the EL layer of the light-emitting element includes a hostmaterial and a guest material (a phosphorescent compound), whereby thelight-emitting element can have an element structure that utilizesphosphorescence as well as fluorescence and element characteristics canbe improved (e.g., see Patent Document 1).

Further, the EL layer includes a hole-injection layer, a hole-transportlayer, a light-emitting layer, an electron-transport layer, anelectron-injection layer, or the like, and includes at least alight-emitting layer. Note that materials suitable for respectivefunctions of these layers have been developed to be applied, wherebyelement characteristics are improved (e.g., see Patent Document 2).

REFERENCE Patent Document 1

-   [Patent Document 1] Japanese Published Patent Application No.    2010-182699-   [Patent Document 2] Japanese Published Patent Application No.    2001-261680

DISCLOSURE OF INVENTION

In order to improve element characteristics of a light-emitting element,it is very important to increase a property of injecting carriers to alight-emitting layer because emission efficiency can be increased. Notethat in the case where the light-emitting layer includes a host materialand a guest material, a magnitude relation of the highest occupiedmolecular orbital level (hereinafter referred to as a HOMO level) of thehost material and the HOMO level of the guest material is considered toaffect a property of injecting carriers (holes) to the light-emittinglayer. This is considered to be because in the case where the HOMO levelof the host material is significantly lower than the HOMO level of theguest material, holes are selectively trapped at an interface on theanode side of the light-emitting layer by the guest material and thusholes are less likely to be distributed in the whole light-emittinglayer. Therefore, it is preferable that the difference between the HOMOlevel of the host material used for the light-emitting layer and theHOMO level of the guest material which is used together with the hostmaterial be small and the host material used for the light-emittinglayer have a high triplet excited energy level (T1 level). By such acombination of the host material and the guest material, carrier balancein a light-emitting layer is favorable; thus, a light-emitting elementhaving high emission efficiency is provided.

In the case where the guest material used for the light-emitting layerhas a high HOMO level, the host material which is used together with theguest material preferably has a high HOMO level in accordance with it.However, in the case of the guest material emitting phosphorescence witha short wavelength, such as blue light, when the HOMO level is high, theT1 level becomes high in accordance with it. Therefore, the use of thematerial satisfying such both conditions is considered to be effectiveto improve emission efficiency. In view of the above, the host materialthat can widely select a phosphorescent compound which is the guestmaterial even if the host material has a higher HOMO level and a higherT1 level than a conventional host material was calculated by quantumchemical calculation and an examination of a molecular structure of thehost material suitable for the light-emitting layer of thelight-emitting element emitting phosphorescence was conducted.

In the design of the molecular structure, the HOMO level can beincreased by an increase in electron density in molecules. However, whenan amine structure or the like is introduced so that the electrondensity is increased, conjugation extends in molecules and thus the T1level is likely to be reduced. In view of the above, the electrondensity is increased by introduction of another 5-membered ring inmolecules, whereby the T1 level is kept high while the HOMO level iskept high.

Accordingly, it was found that an optimal structure of the host materialused for the light-emitting layer of the light-emitting element emittingphosphorescence is a substance containing a 5-membered ring shown by thefollowing general formula (G1), in which a plurality of 5-membered ringsis introduced in molecules in order to increase electron density, whichleads to an increase in the HOMO level, and T1 level.

(In the formula, α¹ to α³ separately represent a substituted orunsubstituted phenylene group or a substituted or unsubstitutedbiphenyldiyl group, Ar¹ to Ar³ separately represent any of a substitutedor unsubstituted phenyl group, a substituted or unsubstituted pyridylgroup, a substituted or unsubstituted pyrimidyl group, a substituted orunsubstituted naphthyl group, a substituted or unsubstituted fluorenylgroup, a substituted or unsubstituted triphenylenyl group, and asubstituted or unsubstituted phenanthrenyl group, and l, m and nseparately are 0 or 1.)

That is, in one embodiment of the present invention, a light-emittinglayer of a light-emitting element includes a first organic compoundwhich is a host material and a second organic compound which is a guestmaterial, and the difference between the HOMO level of the first organiccompound and the HOMO level of the second organic compound is lower thanor equal to 0.3 eV.

Further, in one embodiment of the present invention, a light-emittingelement includes a layer including a first organic compound representedby the above general formula (G1) and a second organic compound which isa guest material between a pair of electrodes. Note that in the abovestructure, the difference between the HOMO level of the first organiccompound and the HOMO level of the second organic compound is lower thanor equal to 0.3 eV.

Further, in the above structure, it is preferable that the secondorganic compound be a phosphorescent compound (an organometallic complexor the like), particularly, a material having a HOMO level higher thanor equal to −5.8 eV.

In each of the above structures, a structure including a hole-injectionlayer, a hole-transport layer, an electron-injection layer, or anelectron-transport layer in addition to the light-emitting layerincluding the first organic compound and the second organic compound canbe employed. At that case, the first organic compound represented by theabove general formula (G1) is a compound having a donor property;therefore, the first organic compound can be used for the hole-injectionlayer or the hole-transport layer.

Further, in each of the above structures, as the first organic compoundrepresented by the above general formula (G1), particularly, an organiccompound represented by the following structural formula (100)(10,15-dihydro-5,10,15-triphenyl-5H-diindolo[3,2-a:3′,2′-c]carbazole(abbreviation: P3Dic)) can be used.

Other embodiments of the present invention are not only a light-emittingdevice including the light-emitting element but also an electronicdevice and a lighting device each including the light-emitting device.The light-emitting device in this specification refers to an imagedisplay device and a light source (e.g., a lighting device). Inaddition, the light-emitting device includes all the following modules:a module in which a connector, such as a flexible printed circuit (FPC)or a tape carrier package (TCP), is attached to a light-emitting device;a module in which a printed wiring board is provided at the end of aTCP; and a module in which an integrated circuit (IC) is directlymounted on a light-emitting element by a chip-on-glass (COG) method.

In one embodiment of the present invention, the difference between theHOMO level of a host material included in a light-emitting layer of alight-emitting element emitting phosphorescence and the HOMO level of aguest material can be lower than or equal to 0.3 eV; therefore, thelight-emitting element having high emission efficiency can be realized.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a concept of one embodiment of the present invention.

FIG. 2 illustrates a structure of a light-emitting element.

FIG. 3 illustrates a structure of a light-emitting element.

FIGS. 4A and 4B illustrate structures of light-emitting elements.

FIGS. 5A and 5B illustrate a light-emitting device.

FIGS. 6A to 6D illustrate electronic devices.

FIGS. 7A to 7C illustrate an electronic device.

FIG. 8 illustrates lighting devices.

FIG. 9 illustrates a structure of a light-emitting element.

FIG. 10 is a graph showing voltage-current characteristics of thelight-emitting element 1, the light-emitting element 2, and thelight-emitting element 3.

FIG. 11 is a graph showing luminance-chromaticity characteristics of thelight-emitting element 1, the light-emitting element 2, and thelight-emitting element 3.

FIG. 12 is a graph showing luminance-external energy efficiencycharacteristics of the light-emitting element 1, the light-emittingelement 2, and the light-emitting element 3.

FIG. 13 shows emission spectra of the light-emitting element 1, thelight-emitting element 2, and the light-emitting element 3.

FIG. 14 is a graph showing voltage-current characteristics of thelight-emitting element 4 and the light-emitting element 5.

FIG. 15 is a graph showing luminance-chromaticity characteristics of thelight-emitting element 4 and the light-emitting element 5.

FIG. 16 is a graph showing luminance-external energy efficiencycharacteristics of the light-emitting element 4 and the light-emittingelement 5.

FIG. 17 shows emission spectra of the light-emitting element 4 and thelight-emitting element 5.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings. Note that the present inventionis not limited to the following description, and various changes andmodifications can be made without departing from the spirit and scope ofthe present invention. Therefore, the present invention should not beconstrued as being limited to the description in the followingembodiments.

Embodiment 1

In this embodiment, a light-emitting element emitting phosphorescence,which is one embodiment of the present invention, will be described.

The light-emitting element described in this embodiment includes, asillustrated in FIG. 1, a first organic compound (a host material) 11 anda second organic compound (a guest material) 12 in a light-emittinglayer 10; the difference between the HOMO level of the first organiccompound (the host material) 11 and the HOMO level of the second organiccompound (the guest material) 12 is lower than or equal to 0.3 eV.

Note that, in injection of holes from a hole-transport layer 13 to thelight-emitting layer 10, in the case where a host material 14 is usedinstead of the first organic compound (the host material) 11 in thelight-emitting layer 10, even if the holes are injected from thehole-transport layer 13 to the host material 14, most of the holes arelikely to enter the level of the second organic compound (the guestmaterial) 12 in the vicinity of an interface between the hole-transportlayer 13 and the light-emitting layer 10 immediately; therefore, aproperty of injecting or transporting holes to the light-emitting layer10 is decreased. Thus, it is considered that the driving voltage islikely to be increased. However, as the host material in thelight-emitting layer 10, in the case where the first organic compound(the host material) 11 having a higher HOMO level than the host material14 (that is, the difference between the HOMO level of the second organiccompound (the guest material) 12 and the HOMO level of the first organiccompound II is small, preferably, lower than or equal to 0.3 eV) isused, holes are likely to enter both of the level of the first organiccompound (the host material) 11 and the level of the second organiccompound (the guest material) 12; therefore, a property of injecting ortransporting holes to the light-emitting layer 10 of the light-emittingelement can be increased.

Here, as a result of calculation of a host material having a higher HOMOlevel and a higher T1 level than the host material 14 by quantumchemical calculation, it was found that as an optimal structure of thefirst organic compound (the host material) 11, a substance containing a5-membered ring shown by the following general formula (G1), in which aplurality of 5-membered rings is introduced in order to increaseelectron density, which leads to an increase in the HOMO level, and T1level which is decreased due to introduction of nitrogen, is preferablyused.

(In the formula, α¹ to α³ separately represent a substituted orunsubstituted phenylene group or a substituted or unsubstitutedbiphenyldiyl group, Ar¹ to Ar³ separately represent any of a substitutedor unsubstituted phenyl group, a substituted or unsubstituted pyridylgroup, a substituted or unsubstituted pyrimidyl group, a substituted orunsubstituted naphthyl group, a substituted or unsubstituted fluorenylgroup, a substituted or unsubstituted triphenylenyl group, and asubstituted or unsubstituted phenanthrenyl group, and l, m and nseparately are 0 or 1.)

In view of the above, the light-emitting element emittingphosphorescence described in this embodiment includes the first organiccompound (the host material) shown by the above general formula (G1) andthe second organic compound (the guest material) in the light-emittinglayer; the difference between the HOMO level of the first organiccompound (the host material) and the HOMO level of the second organiccompound (the guest material) is lower than or equal to 0.3 eV.

Next, the light-emitting element which is one embodiment of the presentinvention will be described with reference to FIG. 2.

The light-emitting element which is one embodiment of the presentinvention has a structure in which, as illustrated in FIG. 2, alight-emitting layer 104 including a first organic compound (a hostmaterial) 105 shown by the above general formula (G1) and a secondorganic compound (a guest material) 106 which is a phosphorescentcompound is interposed between a pair of electrodes (an anode 101 and acathode 102). Note that the light-emitting layer 104 is one offunctional layers included in an EL layer 103 which is in contact withthe pair of electrodes. The EL layer 103 can include not only thelight-emitting layer 104 but also an appropriately selected layer in adesired position, such as a hole-injection layer, a hole-transportlayer, an electron-transport layer, or an electron-injection layer.

Note that specific examples of the first organic compound (the hostmaterial) 105 shown by the above general formula (G1) include thefollowing substances.

In addition, it is preferable that the second organic compound (theguest material) 106 be a phosphorescent compound (an organometalliccomplex or the like), particularly, a material having a HOMO levelhigher than or equal to −5.8 eV.

Note that examples of an organometallic complex which is aphosphorescent compound include tris(2-phenylpyridinato)iridium(III)(abbreviation: Ir(ppy)₃); bis(2-phenylpyridinato)iridium(III)acetylacetonate (abbreviation: Ir(ppy)₂(acac));bis(2-phenylbenzothiazolato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(bt)₂(acac));bis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C^(3′)]iridium(III)acetylacetonate (abbreviation: Ir(btp)₂(acac));(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac));(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(acac));2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)(abbreviation: PtOEP);(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(another name:bis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]);bis(3,5-dimethyl-2-phenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: [Ir(mppr-Me)₂(dpm)]); and(acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III) (anothername:bis[2-(6-methyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: [Ir(mppm)₂(acac)]).

The first organic compound 105 and the second organic compound 106 arenot limited to the above substances as long as the difference betweenthe HOMO level of the first organic compound 105 and the HOMO level ofthe second organic compound 106 is lower than or equal to 0.3 eV.

In the light-emitting element in this embodiment which is one embodimentof the present invention, the difference between the HOMO level of ahost material included in the light-emitting layer of the light-emittingelement emitting phosphorescence and the HOMO level of a guest materialcan be lower than or equal to 0.3 eV; therefore, the light-emittingelement having high emission efficiency can be realized by an increasein the property of injecting holes to the light-emitting layer.

Embodiment 2

In this embodiment, a structure of a light-emitting element which is oneembodiment of the present invention and a manufacturing method thereofare described with reference to FIG. 3.

In a light-emitting element described in this embodiment, as illustratedin FIG. 3, an EL layer 203 including a light-emitting layer 206 isprovided between a pair of electrodes (a first electrode (anode) 201 anda second electrode (cathode) 202), and the EL layer 203 includes ahole-injection layer 204, a hole-transport layer 205, anelectron-transport layer 207, an electron-injection layer 208, and thelike in addition to the light-emitting layer 206.

The light-emitting layer 206 includes a first organic compound 209represented by the following general formula (G1) and a second organiccompound 210, as in the light-emitting element described inEmbodiment 1. Further, the difference between the HOMO level of thefirst organic compound (the host material) 209 and the HOMO level of thesecond organic compound (the guest material) 210 is lower than or equalto 0.3 eV.

Note that, with the structure of the light-emitting layer of thelight-emitting element as described above, a property of injecting holesfrom a hole-transport layer to the light-emitting layer can beincreased, so that emission efficiency of the light-emitting element canbe increased.

Note that the same substances as those in Embodiment 1 can be used asthe first organic compound 209 represented by the following generalformula (G1) and the second organic compound 210 in the above structure,and thus description of specific examples thereof is omitted.

(In the formula, α¹ to α³ separately represent a substituted orunsubstituted phenylene group or a substituted or unsubstitutedbiphenyldiyl group, Ar¹ to Ar³ separately represent any of a substitutedor unsubstituted phenyl group, a substituted or unsubstituted pyridylgroup, a substituted or unsubstituted pyrimidyl group, a substituted orunsubstituted naphthyl group, a substituted or unsubstituted fluorenylgroup, a substituted or unsubstituted triphenylenyl group, and asubstituted or unsubstituted phenanthrenyl group, and l, m and nseparately are 0 or 1.)

The skeleton shown by the above general formula (G1) (thediindolocarbazole skeleton) is a skeleton that is highly planar and hasa high carrier-transport property. In particular, the skeleton has ahigh HOMO level and a high hole-transport property.

Note that in the case where α¹ to α³ and Ar¹ to Ar³ have a substituentin the above general formula (G1), an alkyl group having 1 to 6 carbonatoms is preferable.

Further, in the above general formula (G1), even when α¹ to α³ and Ar¹to Ar³ are bonded to the diindolocarbazole skeleton, electron density isless likely to extend from the diindolocarbazole skeleton to thesesubstituents; therefore, the structure is preferable in that the T1level is not reduced but is maintained.

Furthermore, in the above general formula (G1), it is preferable thatAr¹ to Ar³ separately represent any of a substituted or unsubstitutedpyridyl group and a substituted or unsubstituted pyrimidyl group, inwhich case the substance itself has a bipolar property. In addition,here, it is preferable that l, m, and n be 1 because HOMO-LUMO overlapcan be reduced by the diindolocarbazole skeleton and Ar¹ to Ar³;therefore, the T1 can be kept high.

Further, in the above general formula (G1), it is preferable that α¹ toα³ separately represent any of a substituted or unsubstituted phenylenegroup and a substituted or unsubstituted biphenyldiyl group, Ar¹ to Ar³separately represent any of a substituted or unsubstituted phenyl group,a substituted or unsubstituted pyridyl group, and a substituted orunsubstituted pyrimidyl group, and l, m and n be 0 or 1, in which casethe T1 can be kept high. This is because in the case where α¹ to α³ areeach composed of a six-membered ring, the T1 can be kept higher than inthe case where a higher condensed ring is used.

Further, in the above general formula (G1), it is preferable that α¹ toα³ separately represent any of a substituted or unsubstituted phenylenegroup and a substituted or unsubstituted biphenyldiyl group, Ar¹ to Ar³separately represent any of a substituted or unsubstituted phenyl group,a substituted or unsubstituted naphthyl group, a substituted orunsubstituted fluorenyl group, a substituted or unsubstitutedtriphenylenyl group, and a substituted or unsubstituted phenanthrenylgroup, in which case such a group is a hydrocarbon group; therefore, theHOMO level can be kept high.

Next, a manufacturing method of the light-emitting element described inthis embodiment is specifically described.

For the first electrode (anode) 201 and the second electrode (cathode)202, a metal, an alloy, an electrically conductive compound, a mixturethereof, or the like can be used. Specific examples are indium oxide-tinoxide (indium tin oxide (ITO)), indium oxide-tin oxide containingsilicon or silicon oxide, indium oxide-zinc oxide (indium zinc oxide),indium oxide containing tungsten oxide and zinc oxide, gold (Au),platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum(Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), and titanium(Ti). Other examples are elements that belong to Group 1 or 2 in theperiodic table, for example, an alkali metal such as lithium (Li) orcesium (Cs), an alkaline earth metal such as calcium (Ca) or strontium(Sr), magnesium (Mg), an alloy containing such an element (MgAg orAILi), a rare earth metal such as europium (Eu) or ytterbium (Yb), analloy containing such an element, and graphene. The first electrode(anode) 201 and the second electrode (cathode) 202 can be formed by, forexample, a sputtering method, an evaporation method (including a vacuumevaporation method), or the like.

Examples of a substance having a high hole-transport property which isused for the hole-injection layer 204 and the hole-transport layer 205include aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine(abbreviation: TCTA), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2), and3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1). Alternatively, the following carbazolederivative can be used: 4,4′-di(N-carbazolyl)biphenyl (abbreviation:CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),and 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-Carbazole (abbreviation:CzPA). The substances mentioned here are mainly ones that have a holemobility of 10⁻⁶ cm²/Vs or higher. However, other substances than theabove described materials may also be used as long as the substanceshave higher hole-transport properties than electron-transportproperties.

Further, a polymer such as poly(N-vinylcarbazole) (abbreviation: PVK),poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), orpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD) can be used.

Note that the first organic compound represented by the general formula(G1) is a substance having a high donor property and a highhole-transport property; therefore, the first organic compound can bealso used for the hole-injection layer and the hole-transport layer.

Further, examples of an acceptor substance which can be used for thehole-injection layer 204 include oxides of transition metals, oxides ofmetals belonging to Groups 4 to 8 of the periodic table, and the like.Specifically, molybdenum oxide is particularly preferable.

The light-emitting layer 206 includes the first organic compoundrepresented by the general formula (G1) and the second organic compound,as described above.

The electron-transport layer 207 is a layer that contains a substancehaving a high electron-transport property. For the electron-transportlayer 207, it is possible to use a metal complex such as Alq₃,tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),BAlq, Zn(BOX)₂, or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc(II)(abbreviation: Zn(BTZ)₂). Further, a heteroaromatic compound such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4′-tert-butylphenyl)-4-phenyl-5-(4″-biphenylyl)-1,2,4-triazole(abbreviation: TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs) can alsobe used. Further, a high molecular compound such aspoly(2,5-pyridinediyl) (abbreviation: PPy),poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py) orpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy) can be used. The substances mentioned here aremainly ones that have an electron mobility of 10⁻⁶ cm²/Vs or higher.However, substances other than the above described substances may alsobe used in the electron-transport layer 207 as long as the substanceshave higher electron-transport properties than hole-transportproperties.

The electron-transport layer 207 is not limited to a single layer, andmay be a stack of two or more layers containing any of the abovesubstances.

The electron-injection layer 208 is a layer that contains a substancehaving a high electron-injection property. For the electron-injectionlayer 208, an alkali metal, an alkaline earth metal, or a compoundthereof, such as lithium fluoride (LiF), cesium fluoride (CsF), calciumfluoride (CaF₂), or lithium oxide (LiO_(x)), can be used. Alternatively,a rare earth metal compound like erbium fluoride (ErF₃) can be used. Theabove-mentioned substances for forming the electron-transport layer 207can also be used.

Alternatively, a composite material in which an organic compound and anelectron donor (a donor) are mixed may be used for theelectron-injection layer 208. The composite material is superior in anelectron-injection property and an electron-transport property, sinceelectrons are generated in the organic compound by the electron donor.The organic compound here is preferably a material excellent intransporting the generated electrons, and specifically any of the abovesubstances (such as metal complexes and heteroaromatic compounds) forthe electron-transport layer 207 can be used. As the electron donor, asubstance showing an electron-donating property with respect to theorganic compound may be used. Specifically, an alkali metal, an alkalineearth metal, and a rare earth metal are preferable, and lithium, cesium,magnesium, calcium, erbium, ytterbium, and the like are given. Further,an alkali metal oxide or an alkaline earth metal oxide is preferable,and for example, lithium oxide, calcium oxide, barium oxide, and thelike can be given. Alternatively, Lewis base such as magnesium oxide canalso be used. An organic compound such as tetrathiafulvalene(abbreviation: TTF) can also be used.

Note that the hole-injection layer 204, the hole-transport layer 205,the light-emitting layer 206, the electron-transport layer 207, and theelectron-injection layer 208 which are mentioned above can each beformed by a method such as an evaporation method (including a vacuumevaporation method), an inkjet method, or a coating method.

Light emission obtained in the light-emitting layer 206 of theabove-described light-emitting element is extracted to the outsidethrough either the first electrode 201 or the second electrode 202 orboth. Therefore, either the first electrode 201 or the second electrode202 in this embodiment, or both, is an electrode having alight-transmitting property.

In the light-emitting element described in this embodiment, thelight-emitting layer includes the first organic compound represented bythe above general formula (G1) and the second organic compound, and thedifference between the HOMO level of the first organic compound (thehost material) and the HOMO level of the second organic compound (theguest material) is lower than or equal to 0.3 eV. Therefore, thelight-emitting element having high emission efficiency can be achieved.

Note that the light-emitting element described in this embodiment is oneembodiment of the present invention and is particularly characterized bythe structure of the light-emitting layer. Therefore, when the structuredescribed in this embodiment is employed, a passive matrixlight-emitting device, an active matrix light-emitting device, and thelike can be manufactured. Each of these light-emitting devices isincluded in the present invention.

Note that there is no particular limitation on the structure of the TFTin the case of manufacturing the active matrix light-emitting device.For example, a staggered TFT or an inverted staggered TFT can be used asappropriate. Further, a driver circuit formed over a TFT substrate maybe fanned using both of an n-type TFT and a p-type TFT or only either ann-type TFT or a p-type TFT. Furthermore, there is no particularlimitation on the crystallinity of a semiconductor film used for theTFT. For example, an amorphous semiconductor film, a crystallinesemiconductor film, an oxide semiconductor film, or the like can beused.

Note that the structure described in this embodiment can be used incombination with any of the structures described in the otherembodiments, as appropriate.

Embodiment 3

In this embodiment, as one embodiment of the present invention, alight-emitting element (hereinafter referred to as tandem light-emittingelement) in which a plurality of EL layers are included so as tosandwich a charge-generation layer will be described.

A light-emitting element described in this embodiment is a tandemlight-emitting element including a plurality of EL layers (a first ELlayer 302(1) and a second EL layer 302(2)) between a pair of electrodes(a first electrode 301 and a second electrode 304) as illustrated inFIG. 4A.

In this embodiment, the first electrode 301 functions as an anode, andthe second electrode 304 functions as a cathode. Note that the firstelectrode 301 and the second electrode 304 can have structures similarto those described in Embodiment 1. In addition, although the pluralityof EL layers (the first EL layer 302(1) and the second EL layer 302(2))may have structures similar to those described in Embodiment 1 or 2, anyof the EL layers may have a structure similar to that described inEmbodiment 1 or 2. In other words, the structures of the first EL layer302(1) and the second EL layer 302(2) may be the same or different fromeach other and can be similar to those described in Embodiment 1 or 2.

Further, a charge-generation layer 305 is provided between the pluralityof EL layers (the first EL layer 302(1) and the second EL layer 302(2)).The charge generation layer 305 has a function of injecting electronsinto one of the EL layers and injecting holes into the other of the ELlayers when a voltage is applied to the first electrode 301 and thesecond electrode 304. In this embodiment, when voltage is applied suchthat the potential of the first electrode 301 is higher than that of thesecond electrode 304, the charge-generation layer 305 injects electronsinto the first EL layer 302(1) and injects holes into the second ELlayer 302(2).

Note that in terms of light extraction efficiency, the charge-generationlayer 305 preferably has a light-transmitting property with respect tovisible light (specifically, the charge-generation layer 305 has avisible light transmittance of 40% or more). Further, the chargegeneration layer 305 functions even if it has lower conductivity thanthe first electrode 301 or the second electrode 304.

The charge generation layer 305 may have either a structure in which anelectron acceptor (acceptor) is added to an organic compound having ahigh hole-transport property or a structure in which an electron donor(donor) is added to an organic compound having a high electron-transportproperty. Alternatively, both of these structures may be stacked.

In the case of the structure in which an electron acceptor is added toan organic compound having a high hole-transport property, as theorganic compound having a high hole-transport property, for example, anaromatic amine compound such as NPB, TPD, TDATA, MTDATA, or4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), or the like can be used. The substances mentionedhere are mainly ones that have a hole mobility of 10⁻⁶ cm²/Vs or higher.However, substances other than the above substances may be used as longas they are organic compounds having a hole-transport property higherthan an electron-transport property.

Further, as the electron acceptor,7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and the like can be given. In addition, atransition metal oxide can be given. In addition, an oxide of metalsthat belong to Group 4 to Group 8 of the periodic table can be given.Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromiumoxide, molybdenum oxide, tungsten oxide, manganese oxide, and rheniumoxide are preferable since their electron-accepting property is high.Among these, molybdenum oxide is especially preferable since it isstable in the air and its hygroscopic property is low and is easilytreated.

On the other hand, in the case of the structure in which an electrondonor is added to an organic compound having a high electron-transportproperty, as the organic compound having a high electron-transportproperty, for example, a metal complex having a quinoline skeleton or abenzoquinoline skeleton, such as Alq, Almq₃, BeBq₂, or BAlq, or the likecan be used. Alternatively, a metal complex having an oxazole-basedligand or a thiazole-based ligand, such as Zn(BOX)₂ or Zn(BTZ)₂ can beused. Alternatively, in addition to such a metal complex, PBD, OXD-7,TAZ, BPhen, BCP, or the like can be used. The substances mentioned hereare mainly ones that have an electron mobility of 10⁻⁶ cm²/Vs or higher.Note that substances other than the above substances may be used as longas they are organic compounds having an electron-transport propertyhigher than a hole-transport property.

As the electron donor, it is possible to use an alkali metal, analkaline earth metal, a rare earth metal, a metal belonging to Group 2or 13 of the periodic table, or an oxide or a carbonate thereof.Specifically, lithium (Li), cesium (Cs), magnesium (Mg), calcium (Ca),ytterbium (Yb), indium (In), lithium oxide, cesium carbonate, or thelike is preferably used. Alternatively, an organic compound such astetrathianaphthacene may be used as the electron donor.

Note that forming the charge generation layer 305 by using the abovematerials can suppress an increase in driving voltage caused by thestack of the EL layers.

Although the light-emitting element including two EL layers is describedin this embodiment, the present invention can be similarly applied to alight-emitting element in which n EL layers (302(1) to 302(n)) (n isthree or more) are stacked as illustrated in FIG. 4B. In the case wherea plurality of EL layers are included between a pair of electrodes as inthe light-emitting element according to this embodiment, chargegeneration layers (305(1) to 305(n−1)) are each provided between the ELlayers, whereby light emission in a high luminance region can beobtained while current density is kept low; thus, a light-emittingelement having a long lifetime can be obtained. When the light-emittingelement is applied for illumination, voltage drop due to resistance ofan electrode material can be reduced, thereby achieving homogeneouslight emission in a large area. Moreover, a light-emitting device of lowpower consumption, which can be driven at low voltage, can be achieved.

Further, by forming EL layers to emit light of different colors fromeach other, a light-emitting element as a whole can provide lightemission of a desired color. For example, by forming a light-emittingelement having two EL layers such that the emission color of the firstEL layer and the emission color of the second EL layer are complementarycolors, the light-emitting element can provide white light emission as awhole. Note that the word “complementary” means color relationship inwhich an achromatic color is obtained when colors are mixed. That is,white light emission can be obtained by mixture of light fromsubstances, of which the light emission colors are complementary colors.

Further, the same can be applied to a light-emitting element havingthree EL layers. For example, the light-emitting element as a whole canprovide white light emission when the emission color of the first ELlayer is red, the emission color of the second EL layer is green, andthe emission color of the third EL layer is blue.

In the structure described in this embodiment in which EL layers arestacked with a charge generation layer provided therebetween, byadjusting the distance between electrodes (the first electrode 301 andthe second electrode 304), the light-emitting element can have a microoptical resonator (microcavity) structure utilizing a resonant effect oflight.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 4

In this embodiment, a light-emitting device including a light-emittingelement which is one embodiment of the present invention will bedescribed.

Note that any of the light-emitting elements described in the otherembodiments can be applied to the light-emitting element. Thelight-emitting device can be either a passive matrix light-emittingdevice or an active matrix light-emitting device. In this embodiment, anactive matrix light-emitting device is described with reference to FIGS.5A and 5B.

Note that FIG. 5A is a top view illustrating a light-emitting device andFIG. 5B is a cross-sectional view taken along chain line A-A′ in FIG.5A. The active matrix light-emitting device according to this embodimentincludes a pixel portion 502 provided over an element substrate 501, adriver circuit portion (a source line driver circuit) 503, and drivercircuit portions (gate line driver circuits) 504 a and 504 b. The pixelportion 502, the driver circuit portion 503, and the driver circuitportions 504 a and 504 b are sealed between the element substrate 501and the sealing substrate 506 by a sealant 505.

In addition, there is provided a lead wiring 507 over the elementsubstrate 501. The lead wiring 507 is provided for connecting anexternal input terminal through which a signal (e.g., a video signal, aclock signal, a start signal, and a reset signal) or a potential fromthe outside is transmitted to the driver circuit portion 503 and thedriver circuit portions 504 a and 504 b. Here, an example is describedin which a flexible printed circuit (FPC) 508 is provided as theexternal input terminal. Although only the FPC is illustrated here, aprinted wiring board (PWB) may be attached to the FPC. Thelight-emitting device in the present specification includes, in itscategory, not only the light-emitting device itself but also thelight-emitting device provided with the FPC or the PWB.

Next, a cross-sectional structure is explained with reference to FIG.5B. The driver circuit portion and the pixel portion are formed over theelement substrate 501; here are illustrated the driver circuit portion503 which is the source line driver circuit and the pixel portion 502.

An example is illustrated in which a CMOS circuit which is a combinationof an n-channel TFT 509 and a p-channel TFT 510 is formed as the drivercircuit portion 503. Note that a circuit included in the driver circuitportion may be formed using various CMOS circuits, PMOS circuits, orNMOS circuits. Although a driver integrated type in which the drivercircuit is formed over the substrate is described in this embodiment,the driver circuit may not necessarily be formed over the substrate, andthe driver circuit can be formed outside, not over the substrate.

The pixel portion 502 is formed of a plurality of pixels each of whichincludes a switching TFT 511, a current control TFT 512, and a firstelectrode (anode) 513 which is electrically connected to a wiring (asource electrode or a drain electrode) of the current control TFT 512.Note that an insulator 514 is formed to cover end portions of the firstelectrode (anode) 513. In this embodiment, the insulator 514 is formedusing a positive photosensitive acrylic resin.

In addition, in order to obtain favorable coverage by a film which is tobe stacked over the insulator 514, the insulator 514 is preferablyformed so as to have a curved surface with curvature at an upper edgeportion or a lower edge portion. For example, in the case of using apositive photosensitive acrylic resin as a material for the insulator514, the insulator 514 is preferably formed so as to have a curvedsurface with a curvature radius (0.2 μm to 3 μm) at the upper edgeportion. Note that the insulator 514 can be formed using either anegative photosensitive resin or a positive photosensitive resin. It ispossible to use, without limitation to an organic compound, either anorganic compound or an inorganic compound such as silicon oxide orsilicon oxynitride.

A light-emitting element 517 is formed by stacking an EL layer 515 and asecond electrode (cathode) 516 over the first electrode (anode) 513. TheEL layer 515 includes at least the light-emitting layer described inEmbodiment 1. Further, in the EL layer 515, a hole-injection layer, ahole-transport layer, an electron-transport layer, an electron-injectionlayer, a charge-generation layer, and the like can be provided asappropriate in addition to the light-emitting layer.

For the first electrode (anode) 513, the EL layer 515, and the secondelectrode (cathode) 516, the materials described in Embodiment 2 can beused. Although not illustrated, the second electrode (cathode) 516 iselectrically connected to the FPC 508 which is the external inputterminal.

In addition, although the cross-sectional view of FIG. 5B illustratesonly one light-emitting element 517, a plurality of light-emittingelements are arranged in matrix in the pixel portion 502. Light-emittingelements that emit light of three kinds of colors (R, and B) areselectively formed in the pixel portion 502, whereby a light-emittingdevice capable of full color display can be obtained. Alternatively, alight-emitting device which is capable of full color display may bemanufactured by a combination with color filters.

Further, the sealing substrate 506 is attached to the element substrate501 with the sealant 505, whereby a light-emitting element 517 isprovided in a space 518 surrounded by the element substrate 501, thesealing substrate 506, and the sealant 505. Note that the space 518 maybe filled with an inert gas (such as nitrogen and argon) or the sealant505.

An epoxy-based resin is preferably used for the sealant 505. A materialused for these is desirably a material which does not transmit moistureor oxygen as possible. As the sealing substrate 506, a plastic substrateformed of fiberglass-reinforced plastics (FRP), polyvinyl fluoride(PVF), polyester, acrylic, or the like can be used besides a glasssubstrate or a quartz substrate.

As described above, an active matrix light-emitting device can beobtained.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 5

In this embodiment, examples of a variety of electronic devices whichare completed using a light-emitting device, which is fabricated using alight-emitting element of one embodiment of the present invention, aredescribed with reference to FIGS. 6A to 6D and FIGS. 7A to 7C.

Examples of the electronic devices to which the light-emitting device isapplied are a television device (also referred to as television ortelevision receiver), a monitor of a computer or the like, a camera suchas a digital camera or a digital video camera, a digital photo frame, amobile phone (also referred to as cellular phone or cellular phonedevice), a portable game machine, a portable information terminal, anaudio reproducing device, a large-sized game machine such as a pachinkomachine, and the like. Specific examples of these electronic devices areshown in FIGS. 6A to 6D.

FIG. 6A illustrates an example of a television device. In the televisiondevice 7100, a display portion 7103 is incorporated in a housing 7101.Images can be displayed by the display portion 7103, and thelight-emitting device can be used for the display portion 7103. Inaddition, here, the housing 7101 is supported by a stand 7105.

The television device 7100 can be operated by an operation switch of thehousing 7101 or a separate remote controller 7110. With operation keys7109 of the remote controller 7110, channels and volume can becontrolled and images displayed on the display portion 7103 can becontrolled. Furthermore, the remote controller 7110 may be provided witha display portion 7107 for displaying data output from the remotecontroller 7110.

Note that the television device 7100 is provided with a receiver, amodem, and the like. With the receiver, a general television broadcastcan be received. Furthermore, when the television device 7100 isconnected to a communication network by wired or wireless connection viathe modem, one-way (from a transmitter to a receiver) or two-way(between a transmitter and a receiver, between receivers, or the like)data communication can be performed.

FIG. 6B illustrates a computer, which includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnection port 7205, a pointing device 7206, and the like. Note thatthis computer is manufactured using the light-emitting device for thedisplay portion 7203.

FIG. 6C illustrates a portable game machine having two housings, ahousing 7301 and a housing 7302, which are connected with a jointportion 7303 so that the portable game machine can be opened or folded.A display portion 7304 is incorporated in the housing 7301 and a displayportion 7305 is incorporated in the housing 7302. In addition, theportable game machine illustrated in FIG. 6C includes a speaker portion7306, a recording medium insertion portion 7307, an LED lamp 7308, aninput unit (an operation key 7309, a connection terminal 7310, a sensor7311 (sensor having a function of measuring force, displacement,position, speed, acceleration, angular velocity, rotational frequency,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, current, voltage, electric power,radiation, flow rate, humidity, gradient, oscillation, odor, or infraredrays), or a microphone 7312), and the like. It is needless to say thatthe structure of the portable game machine is not limited to the aboveas long as a light-emitting device is used for at least either thedisplay portion 7304 or the display portion 7305, or both, and mayinclude other accessories as appropriate. The portable game machineillustrated in FIG. 6C has a function of reading a program or datastored in a recording medium to display it in the display portion, and afunction of sharing information with another portable game machine bywireless communication. Note that the functions of the portable gamemachine illustrated in FIG. 6C are not limited to these functions, andthe portable game machine can have various functions.

FIG. 6D illustrates an example of a mobile phone. The mobile phone 7400is provided with a display portion 7402 incorporated in a housing 7401,operation buttons 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400is manufactured using a light-emitting device for the display portion7402.

When the display portion 7402 of the mobile phone 7400 illustrated inFIG. 6D is touched with a finger or the like, data can be input into themobile phone 7400. Further, operations such as making a call andcreating an e-mail can be performed by touching the display portion 7402with a finger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in the case of making a call or creating an e-mail, a textinput mode mainly for inputting text is selected for the display portion7402 so that text displayed on a screen can be inputted. In this case,it is preferable to display a keyboard or number buttons on almost theentire screen of the display portion 7402.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside themobile phone 7400, display on the screen of the display portion 7402 canbe automatically changed by determining the orientation of the mobilephone 7400 (whether the mobile phone is placed horizontally orvertically for a landscape mode or a portrait mode).

The screen modes are switched by touching the display portion 7402 oroperating the operation buttons 7403 of the housing 7401. Alternatively,the screen modes can be switched depending on kinds of images displayedon the display portion 7402. For example, when a signal of an imagedisplayed on the display portion is a signal of moving image data, thescreen mode is switched to the display mode. When the signal is a signalof text data, the screen mode is switched to the input mode.

Moreover, in the input mode, when input by touching the display portion7402 is not performed within a specified period while a signal detectedby an optical sensor in the display portion 7402 is detected, the screenmode may be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken when thedisplay portion 7402 is touched with the palm or the finger, wherebypersonal authentication can be performed. Further, by providing abacklight or a sensing light source which emits a near-infrared light inthe display portion, an image of a finger vein, a palm vein, or the likecan be taken.

FIGS. 7A and 7B illustrate a tablet terminal that can be folded. In FIG.7A, the tablet terminal is opened, and includes a housing 9630, adisplay portion 9631 a, a display portion 9631 b, a switch 9034 forswitching display modes, a power button 9035, a switch 9036 forswitching to power-saving mode, a clip 9033, and an operation button9038. The tablet terminal is manufactured using the light-emittingdevice for either the display portion 9631 a or the display portion 9631b or both.

Part of the display portion 9631 a can be a touch panel area 9632 a anddata can be input when a displayed operation key 9637 is touched. Notethat FIG. 10 shows, as an example, that half of the area of the displayportion 9631 a has only a display function and the other half of thearea has a touch panel function. However, the structure of the displayportion 9631 a is not limited to this, and all the area of the displayportion 9631 a may have a touch panel function. For example, all thearea of the display portion 9631 a can display keyboard buttons andserve as a touch panel while the display portion 9631 b can be used as adisplay screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touch panel area 9632 b. When a finger, a stylus, or the liketouches the place where a button 9639 for switching to keyboard displayis displayed in the touch panel, keyboard buttons can be displayed onthe display portion 9631 b.

Touch input can be performed concurrently on the touch panel areas 9632a and 9632 b.

The switch 9034 for switching display modes can switch displayorientation (e.g., between landscape mode and portrait mode) and selecta display mode (switch between monochrome display and color display),for example. With the switch 9036 for switching to power-saving mode,the luminance of display can be optimized in accordance with the amountof external light at the time when the tablet terminal is in use, whichis detected with an optical sensor incorporated in the tablet terminal.The tablet terminal may include another detection device such as asensor for detecting orientation (e.g., a gyroscope or an accelerationsensor) in addition to the optical sensor.

Although the display portion 9631 a and the display portion 9631 b havethe same display area in FIG. 7A, one embodiment of the presentinvention is not limited to this example. The display portion 9631 a andthe display portion 9631 b may have different areas or different displayquality. For example, one of them may be a display panel that candisplay higher-definition images than the other.

FIG. 7B illustrates the tablet terminal folded, which includes thehousing 9630, a solar battery 9633, a charge and discharge controlcircuit 9634, a battery 9635, and a DCDC converter 9636. Note that FIG.7B shows an example in which the charge and discharge control circuit9634 includes the battery 9635 and the DCDC converter 9636.

Since the tablet terminal can be folded in two, the housing 9630 can beclosed when the tablet terminal is not in use. Thus, the displayportions 9631 a and 9631 b can be protected, thereby providing a tabletterminal with high endurance and high reliability for long-term use.

The tablet terminal illustrated in FIGS. 7A and 7B can have otherfunctions such as a function of displaying various kinds of data (e.g.,a still image, a moving image, and a text image), a function ofdisplaying a calendar, a date, the time, or the like on the displayportion, a touch-input function of operating or editing the datadisplayed on the display portion by touch input, and a function ofcontrolling processing by various kinds of software (programs).

The solar battery 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that a structure in whichthe solar battery 9633 is provided on one or both surfaces of thehousing 9630 is preferable because the battery 9635 can be chargedefficiently. When a lithium ion battery is used as the battery 9635,there is an advantage of downsizing or the like.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 7B are described with reference to a blockdiagram of FIG. 7C. FIG. 7C illustrates the solar battery 9633, thebattery 9635, the DCDC converter 9636, a converter 9638, switches SW1 toSW3, and the display portion 9631. The battery 9635, the DCDC converter9636, the converter 9638, and the switches SW1 to SW3 correspond to thecharge and discharge control circuit 9634 in FIG. 7B.

First, an example of operation in the case where power is generated bythe solar battery 9633 using external light is described. The voltage ofpower generated by the solar battery is raised or lowered by the DCDCconverter 9636 so that the power has a voltage for charging the battery9635. When the display portion 9631 is operated with the power from thesolar battery 9633, the switch SW1 is turned on and the voltage of thepower is raised or lowered by the converter 9638 to a voltage needed foroperating the display portion 9631. In addition, when display on thedisplay portion 9631 is not performed, the switch SW1 may be turned offand the switch SW2 may be turned on so that the battery 9635 is charged.

Here, the solar battery 9633 is shown as an example of a powergeneration means; however, there is no particular limitation on a way ofcharging the battery 9635, and the battery 9635 may be charged withanother power generation means such as a piezoelectric element or athermoelectric conversion element (Peltier element). For example, thebattery 9635 may be charged with a non-contact power transmission modulethat transmits and receives power wirelessly (without contact) to chargethe battery or with a combination of other charging means.

It is needless to say that one embodiment of the present invention isnot limited to the electronic device illustrated in FIGS. 7A to 7C aslong as the display portion described in the above embodiment isincluded.

As described above, the electronic devices can be obtained byapplication of the light-emitting device according to one embodiment ofthe present invention. The light-emitting device has a remarkably wideapplication range, and can be applied to electronic devices in a varietyof fields.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 6

In this embodiment, examples of a lighting device to which alight-emitting device including a light-emitting element of oneembodiment of the present invention is applied, are described withreference to FIG. 8.

FIG. 8 illustrates an example in which the light-emitting device is usedas an indoor lighting device 8001. Note that since the area of thelight-emitting device can be increased, a lighting device having a largearea can also be formed. In addition, a lighting device 8002 in which alight-emitting region has a curved surface can also be obtained with theuse of a housing with a curved surface. A light-emitting elementincluded in the light-emitting device described in this embodiment is ina thin film form, which allows the housing to be designed more freely.Therefore, the lighting device can be elaborately designed in a varietyof ways. Further, a wall of the room may be provided with a large-sizedlighting device 8003.

Moreover, when the light-emitting device is used for a table by beingused as a surface of a table, a lighting device 8004 which has afunction as a table can be obtained. When the light-emitting device isused as part of other furniture, a lighting device which has a functionas the furniture can be obtained.

In this manner, a variety of lighting devices to which thelight-emitting device is applied can be obtained. Note that suchlighting devices are also embodiments of the present invention.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Example 1

In this example, light-emitting elements which are each one embodimentof the present invention are fabricated, and the measurement results ofthe characteristics thereof are shown. Note that a light-emittingelement 1 in this example is a comparative light-emitting element forcomparison with a light-emitting element 2 and a light-emitting element3. In this example, in light-emitting layers of the light-emittingelements, the HOMO level of a host material(10,15-dihydro-5,10,15-triphenyl-5H-diindolo[3,2-a:3′,2′-c]carbazole(abbreviation: P3Dic)) which is used for each of the light-emittingelement 2 and the light-emitting element 3 is higher than the HOMO levelof a host material (9-phenyl-9H-3-(9-phenyl-9H-carbazol-3-yl)carbazole(abbreviation: PCCP)) used for the light-emitting element 1. That is, inthe light-emitting element 2 and the light-emitting element 3, thedifference between the HOMO level of the host material (P3Dic(abbreviation)) and the HOMO level of the guest material([Ir(mpptz-dmp)₃] (abbreviation)) is lower than or equal to 0.3 eV; inthe light-emitting element 1 which is a comparative light-emittingelement, the difference is higher than 0.3 eV. The light-emittingelements fabricated in this example are described with reference to FIG.9. Chemical formulae of materials used in this example are shown below.

[Fabrication of Light-Emitting Element 1, Light-Emitting Element 2, andLight-Emitting Element 3]

First, a film of indium oxide-tin oxide containing silicon oxide (ITSO)was formed over a glass substrate 1100 by a sputtering method, so that afirst electrode 1101 functioning as an anode was formed. The thicknesswas 110 nm and the electrode area was 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, the surface of the substrate was washed, baked at 200°C. for one hour, and subjected to UV ozone treatment for 370 seconds.

After that, the substrate 1100 was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and subjected to vacuum baking at 170° C. for 30 minutes in a heatingchamber of the vacuum evaporation apparatus, and then the substrate 1100was cooled down for about 30 minutes.

Next, the substrate 1100 was fixed to a holder provided in the vacuumevaporation apparatus so that a surface of the substrate 1100 over whichthe first electrode 1101 was formed faced downward. In this example, acase will be described in which a hole-injection layer 1111, ahole-transport layer 1112, a light-emitting layer 1113, anelectron-transport layer 1114, and an electron-injection layer 1115which are included in an EL layer 1102 are sequentially formed by avacuum evaporation method.

After reducing the pressure of the vacuum evaporation apparatus to 10⁻⁴Pa, 1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation: DBT3P-II) andmolybdenum oxide(VI) were co-evaporated with a mass ratio of DBT3P-II(abbreviation) to molybdenum oxide being 4:2, whereby the hole-injectionlayer 1111 was formed over the first electrode 1101. The thickness wasset to 60 nm. Note that a co-evaporation method is an evaporation methodin which a plurality of different substances is concurrently vaporizedfrom respective different evaporation sources.

Then, in the case of the light-emitting element 1,9-phenyl-9H-3-(9-phenyl-9H-carbazol-3-yl)carbazole (abbreviation: PCCP)was deposited by evaporation to a thickness of 20 nm, whereby thehole-transport layer 1112 was formed. In the case of the light-emittingelement 2 and the light-emitting element 3,10,15-dihydro-5,10,15-triphenyl-5H-diindolo[3,2-a:3′,2′-c]carbazole(abbreviation: P3Dic) was deposited by evaporation to a thickness of 20nm, whereby the hole-transport layer 1112 was formed.

Next, the light-emitting layer 1113 was formed over the hole-transportlayer 1112.

For the light-emitting element 1, PCCP (abbreviation),3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation: 35DCzPPy),andtris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III)(abbreviation: [Ir(mpptz-dmp)₃]) were co-evaporated to a thickness of 30nm with a mass ratio of PCCP (abbreviation) to 35DCzPPy (abbreviation)and [Ir(mpptz-dmp)₃] (abbreviation) being 1:0.3:0.06 to form a firstlight-emitting layer 1113 a, and then further co-evaporated to athickness of 10 nm with a mass ratio of 35DCzPPy (abbreviation) to[Ir(mpptz-dmp)₃] (abbreviation) being 1:0.06 to form a secondlight-emitting layer 1113 b; thus, the light-emitting layer 1113 whichis a stacked structure of the first light-emitting layer 1113 a and thesecond light-emitting layer 1113 b was formed.

For the light-emitting element 2, P3Dic (abbreviation), 35DCzPPy(abbreviation), and [Ir(mpptz-dmp)₃] (abbreviation) were co-evaporatedto a thickness of 30 nm with a mass ratio of P3Dic (abbreviation) to35DCzPPy (abbreviation) and [Ir(mpptz-dmp)₃] (abbreviation) being1:0.3:0.06 to form a first light-emitting layer 1113 a, and then furtherco-evaporated to a thickness of 10 nm with a mass ratio of 35DCzPPy(abbreviation) to Ir(mpptz-dmp)₃ (abbreviation) being 1:0.06 to faun asecond light-emitting layer 1113 b; thus, the light-emitting layer 1113which is a stacked structure of the first light-emitting layer 1113 aand the second light-emitting layer 1113 b was formed.

For the light-emitting element 3, P3Dic (abbreviation), 35DCzPPy(abbreviation), and [Ir(mpptz-dmp)₃] (abbreviation) were co-evaporatedto a thickness of 30 nm with a mass ratio of P3Dic (abbreviation) to35DCzPPy (abbreviation) and [Ir(mpptz-dmp)₃] (abbreviation) being0.3:1:0.06 to form a first light-emitting layer 1113 a, and then furtherco-evaporated to a thickness of 10 nm with a mass ratio of 35DCzPPy(abbreviation) to Ir(mpptz-dmp)₃ (abbreviation) being 1:0.06 to form asecond light-emitting layer 1113 b; thus, the light-emitting layer 1113which is a stacked structure of the first light-emitting layer 1113 aand the second light-emitting layer 1113 b was foamed.

Next, after evaporating2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II) to a thickness of 10 nm over thelight-emitting layer 1113, bathophenanthroline (abbreviation: BPhen) wasfurther evaporated to a thickness of 15 nm, whereby theelectron-transport layer 1114 having a stacked structure was formed.Furthermore, lithium fluoride was evaporated to a thickness of 1 nm overthe electron-transport layer 1114, whereby the electron-injection layer1115 was formed.

Finally, aluminum was deposited by evaporation to a thickness of 200 nmover the electron-injection layer 1115 to form the second electrode 1103serving as a cathode; thus, the light-emitting element 1, thelight-emitting element 2, and the light-emitting element 3 wereobtained. Note that, in the above evaporation process, evaporation wasall performed by a resistance heating method.

Table 1 shows element structures of the light-emitting element 1, thelight-emitting element 2, and the light-emitting element 3.

TABLE 1 Hole- Hole- Light- Electron- First injection transport emittinginjection Second electrode layer layer layer Electron-transport layerlayer electrode Light- ITSO DBT3P-II:MoOx PCCP * **** mDBTBIm-II BphenLiF Al emitting (110 nm) (4:2 60 nm) (20 nm) (10 nm) (15 nm) (1 nm) (200nm) element 1 Light- P3Dic ** emitting (20 nm) element 2 Light- ***emitting element 3 * PCCP:35DCzPPy:[Ir(mpptz-dmp)₃] (1:0.3:0.06 30 nm)** P3Dic:35DCzPPy:[Ir(mpptz-dmp)₃] (1:0.3:0.06 30 nm) ***P3Dic:35DCzPPy:[Ir(mpptz-dmp)₃] (0.3:1:0.06 30 nm) ****35DCzPPy:[Ir(mpptz-dmp)₃] (1:0.06 10 nm)

Further, the light-emitting element 1 fabricated, the light-emittingelement 2 fabricated, and the light-emitting element 3 fabricated weresealed in a glove box containing a nitrogen atmosphere so as not to beexposed to the air (specifically, a sealant was applied onto outer edgesof the elements and heat treatment was performed at 80° C. for 1 hour atthe time of sealing).

[Operation Characteristics of Light-Emitting Element 1, Light-EmittingElement 2, and Light-Emitting Element 3]

Operation characteristics of the light-emitting element 1 fabricated,the light-emitting element 2 fabricated, and the light-emitting element3 fabricated were measured. Note that the measurement was carried out atroom temperature (in an atmosphere kept at 25° C.).

FIG. 10 shows voltage-current characteristics of the light-emittingelement 1, the light-emitting element 2, and the light-emitting element3. In FIG. 10, the vertical axis represents current (mA) and thehorizontal axis represents voltage (V). Further, FIG. 11 showsluminance-chromaticity characteristics of the light-emitting element 1,the light-emitting element 2, and the light-emitting element 3. In FIG.11, the vertical axis represents chromaticity coordinate and thehorizontal axis represents luminance (cd/m²). FIG. 12 showsluminance-external energy efficiency characteristics of thelight-emitting element 1, the light-emitting element 2, and thelight-emitting element 3. In FIG. 12, the vertical axis representsexternal energy efficiency (%) and the horizontal axis representsluminance (cd/m²).

According to FIG. 10, it is found that the light-emitting element 2 andthe light-emitting element 3 which are each one embodiment of thepresent invention have lower driving voltage than the light-emittingelement 1 which is a comparative light-emitting element. In addition,according to FIG. 12, it is found that the light-emitting element 2 andthe light-emitting element 3 have higher external energy efficiency thanthe light-emitting element 1. This is considered to be because P3Dic(abbreviation) used for the light-emitting layer of each of thelight-emitting element 2 and the light-emitting element 3 has a higherHOMO level than PCCP used for the light-emitting layer of thelight-emitting element 1 (has a HOMO level close to [Ir(mpptz-dmp)₃](abbreviation)) and also has a high T1 level, whereby the light-emittingelement 2 and the light-emitting element 3 have low driving voltage andthe luminous efficiency is improved. The above reason is supported bythat the light-emitting element 2 containing more P3Dic (abbreviation)in the light-emitting layer than the light-emitting element 3 has lowerdriving voltage and higher efficiency than the light-emitting element 3.

In addition, according to FIG. 11, it is found that the chromaticity ofeach of the light-emitting element 2 and the light-emitting element 3 issubstantially the same as that of the light-emitting element 1.Therefore, it is found that characteristics of the light-emittingelement 2 and the light-emitting element 3 which are one embodiment ofthe present invention can be improved as compared to those of thelight-emitting element 1 while the chromaticity that is substantiallythe same as that of the light-emitting element 1 is maintained. Further,the above results indicate that the light-emitting element 2 and thelight-emitting element 3 have almost no color change at each luminanceand therefore have a favorable carrier balance.

Table 2 shows initial values of main characteristics of thelight-emitting element 1, the light-emitting element 2, and thelight-emitting element 3 at a luminance of about 1000 cd/m².

TABLE 2 Current Current Power External External Voltage Current densityChromaticity Luminance efficiency efficiency quantum energy (V) (mA)(mA/cm²) (x, y) (cd/m²) (cd/A) (lm/W) efficiency (%) efficiency (%)Light- 4.6 0.093 2.3 (0.21, 0.38) 850 37 25 15 8.0 emitting element 1Light- 3.6 0.12 2.9 (0.21, 0.39) 960 33 29 14 9.2 emitting element 2Light- 4 0.13 3.3 (0.22, 0.41) 1100 34 27 13 8.0 emitting element 3

FIG. 13 shows emission spectra of the light-emitting element 1, thelight-emitting element 2, and the light-emitting element 3 which wereobtained by application of a current of 0.1 mA. As shown in FIG. 13, itis found that each of the light-emitting element 1, the light-emittingelement 2, and the light-emitting element 3 has peaks of emissionspectrum at around 481 nm and at around 508 nm; these peaks derive fromthe emission of [Ir(mpptz-dmp)₃] (abbreviation) in the light-emittinglayer 1113.

It was thus found that P3Dic (abbreviation) has a sufficiently high T1level to use as a blue host material. That is, P3Dic (abbreviation) canbe used as a host material in a light-emitting layer of a phosphorescentlight-emitting element having an emission peak in a visible region.

Example 2

In this example, light-emitting elements which are each one embodimentof the present invention are fabricated, and the measurement results ofthe characteristics thereof are shown. Note that a light-emittingelement 4 in this example is a comparative light-emitting element forcomparison with a light-emitting element 5. Note that FIG. 9, which isused for the description of the light-emitting element 1, thelight-emitting element 2, and the light-emitting element 3 in Example 1,is used for describing the light-emitting element 4 and thelight-emitting element 5 in this example. Chemical formulae of materialsused in this example are shown below.

[Fabrication of Light-Emitting Element 4 and Light-Emitting Element 5]

First, indium tin oxide containing silicon oxide (ITSO) was depositedover a glass substrate 1100 by a sputtering method, so that a firstelectrode 1101 functioning as an anode was formed. The thickness was 110nm and the electrode area was 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, the surface of the substrate was washed, baked at 200°C. for one hour, and subjected to UV ozone treatment for 370 seconds.

After that, the substrate 1100 was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and subjected to vacuum baking at 170° C. for 30 minutes in a heatingchamber of the vacuum evaporation apparatus, and then the substrate 1100was cooled down for about 30 minutes.

Next, the substrate 1100 was fixed to a holder provided in the vacuumevaporation apparatus so that a surface of the substrate 1100 over whichthe first electrode 1101 was formed faced downward. In this example, acase will be described in which a hole-injection layer 1111, ahole-transport layer 1112, a light-emitting layer 1113, anelectron-transport layer 1114, and an electron-injection layer 1115which are included in an EL layer 1102 are sequentially formed by avacuum evaporation method.

After reducing the pressure of the vacuum evaporation apparatus to 10⁻⁴Pa, 1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation: DBT3P-II) andmolybdenum oxide(VI) were co-evaporated with a mass ratio of DBT3P-II(abbreviation) to molybdenum oxide being 4:2, whereby the hole-injectionlayer 1111 was formed over the first electrode 1101. The thickness was60 nm. Note that a co-evaporation method is an evaporation method inwhich a plurality of different substances is concurrently vaporized fromrespective different evaporation sources.

Next, 9-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-9H-carbazole (abbreviation:mCzFLP) was evaporated to a thickness of 20 nm, so that thehole-transport layer 1112 was formed.

Next, the light-emitting layer 1113 was formed over the hole-transportlayer 1112.

For the light-emitting element 4,4,6-bis[3-(dibenzothiophen-4-yl)phenyl]pyrimidine (abbreviation:4,6mDBTP2Pm-II), PCCP (abbreviation), andtris(2-phenylpyridinato)iridium(III) (abbreviation: [Ir(ppy)₃]) wereco-evaporated to a thickness of 20 nm with a mass ratio of4,6mDBTP2Pm-II (abbreviation) to PCCP (abbreviation) and [Ir(ppy)₃](abbreviation) being 0.7:0.3:0.06 to form a first light-emitting layer1113 a, and then further co-evaporated to a thickness of 20 nm with amass ratio of 4,6mDBTP2Pm-II (abbreviation) to PCCP (abbreviation) and[Ir(ppy)₃] (abbreviation) being 0.8:0.2:0.06 to form a secondlight-emitting layer 1113 b; thus, the light-emitting layer 1113 whichis a stacked structure of the first light-emitting layer 1113 a and thesecond light-emitting layer 1113 b was formed.

For the light-emitting element 5,4,6-bis[3-(dibenzothiophen-4-yl)phenyl]pyrimidine (abbreviation:4,6mDBTP2Pm-II),10,15-dihydro-5,10,15-triphenyl-5H-diindolo[3,2-a:3′,2′-c]carbazole(abbreviation: P3Dic), and tris(2-phenylpyridinato)iridium(III)(abbreviation: [Ir(ppy)₃]) were co-evaporated to a thickness of 20 nmwith a mass ratio of 4,6mDBTP2Pm-II (abbreviation) to P3Dic(abbreviation) and [Ir(ppy)₃] (abbreviation) being 0.7:0.3:0.06 to forma first light-emitting layer 1113 a, and then further co-evaporated to athickness of 20 nm with a mass ratio of 4,6mDBTP2Pm-II (abbreviation) toP3Dic (abbreviation) and [Ir(ppy)₃] (abbreviation) being 0.8:0.2:0.06 toform a second light-emitting layer 1113 b; thus, the light-emittinglayer 1113 which is a stacked structure of the first light-emittinglayer 1113 a and the second light-emitting layer 1113 b was formed.

Then, 4,6mDBTP2Pm-II (abbreviation) was evaporated to a thickness of 10nm over the light-emitting layer 1113 and bathophenanthroline(abbreviation: Bphen) was evaporated to a thickness of 20 nm, wherebythe electron-transport layer 1114 having a stacked structure was formed.Furthermore, lithium fluoride was evaporated to a thickness of 1 nm overthe electron-transport layer 1114, whereby the electron-injection layer1115 was formed.

Finally, aluminum was evaporated to a thickness of 200 nm over theelectron-injection layer 1115 to form a second electrode 1103 serving asa cathode; thus, the light-emitting element 4 and the light-emittingelement 5 were obtained. Note that, in the above evaporation process,evaporation was all performed by a resistance heating method.

Table 3 shows element structures of the light-emitting element 4 and thelight-emitting element 5.

TABLE 3 Hole- Light- Electron- First Hole- transport emitting injectionSecond electrode injection layer layer layer Electron-transport layerlayer electrode Light- ITSO DBT3P-II:MoOx mCzFLP * 4,6mDBTP2Pm-II BphenLiF Al emitting (110 nm) (4:2 60 nm) (20 nm) (10 nm) (20 nm) (1 nm) (200nm) element 4 Light- ** emitting element 5 *4,6mDBTP2Pm-II:PCCP:[Ir(ppy)₃] (0.7:0.3:0.06 20 nm\0.8:0.2:0.06 20 nm)** 4,6mDBTP2Pm-II:P3Dic:[Ir(ppy)₃] (0.7:0.3:0.06 20 nm\0.8:0.2:0.06 20nm)

Further, the light-emitting element 4 fabricated and the light-emittingelement 5 fabricated were sealed in a glove box containing a nitrogenatmosphere so as not to be exposed to the air (specifically, a sealantwas applied onto outer edges of the elements and heat treatment wasperformed at 80° C. for 1 hour at the time of sealing).

[Operation Characteristics of Light-Emitting Element 4 andLight-Emitting Element 5]

Operation characteristics of the light-emitting element 4 fabricated andthe light-emitting element 5 fabricated were measured. Note that themeasurement was carried out at room temperature (in an atmosphere keptat 25° C.).

FIG. 14 shows voltage-current characteristics of the light-emittingelement 4 and the light-emitting element 5. In FIG. 14, the verticalaxis represents current (mA) and the horizontal axis represents voltage(V). Further, FIG. 15 shows luminance-chromaticity characteristics ofthe light-emitting element 4 and the light-emitting element 5. In FIG.15, the vertical axis represents chromaticity coordinate and thehorizontal axis represents luminance (cd/m²). FIG. 16 showsluminance-external energy efficiency characteristics of thelight-emitting element 4 and the light-emitting element 5. In FIG. 16,the vertical axis represents external energy efficiency (%) and thehorizontal axis represents luminance (cd/m²).

According to FIG. 16, it is found that the light-emitting element 5 hashigher external energy efficiency than the light-emitting element 4.This is considered to be because P3Dic used for the light-emitting layerof the light-emitting element 5 has a higher HOMO level and a highercarrier-transport property than PCCP used for the light-emitting layerof the light-emitting element 4; therefore, a recombination region isenlarged and a carrier balance is improved in the light-emitting element5.

In addition, according to FIG. 15, it is found that the chromaticity ofthe light-emitting element 5 is substantially the same as that of thelight-emitting element 4. Therefore, it is found that characteristics ofthe light-emitting element 5 which is one embodiment of the presentinvention can be improved as compared to those of the light-emittingelement 4 while the chromaticity that is substantially the same as thatof the light-emitting element 4 is maintained.

Table 4 shows initial values of main characteristics of thelight-emitting element 4 and the light-emitting element 5 at a luminanceof about 1000 cd/m².

TABLE 4 Current Current Power External External Voltage Current densityChromaticity Luminance efficiency efficiency quantum energy (V) (mA)(mA/cm²) (x, y) (cd/m²) (cd/A) (lm/W) efficiency (%) efficiency (%)Light- 3.6 0.07 1.7 (0.32, 0.61) 850 49 43 14 9.2 emitting element 4Light- 3.6 0.08 1.9 (0.32, 0.61) 990 51 45 15 9.8 emitting element 5

FIG. 17 shows emission spectra of the light-emitting element 4 and thelight-emitting element 5 which were obtained by application of a currentof 0.1 mA. As shown in FIG. 17, each of the light-emitting element 4 andthe light-emitting element has peaks of emission spectrum at around 517nm and at around 544 nm; these peak derive from the emission of[Ir(ppy)₃] (abbreviation) in the light-emitting layer 1113. Further, theabove results indicate that the light-emitting element 4 and thelight-emitting element 5 have almost no color change at each luminanceand therefore have a favorable carrier balance.

Example 3

In this example, the HOMO levels, the LUMO levels, and the T1 levels of10,15-dihydro-5,10,15-triphenyl-5H-diindolo[3,2-a:3′,2′-c]carbazole(abbreviation: P3Dic (structural formula (100))) which is one embodimentof the present invention;10,15-dihydro-5,10,15-tribiphenyl-5H-diindolo[3,2-a:3′,2′-c]carbazole(abbreviation: BP3Dic (structural formula (104))); compounds representedby the following structural formulae (structural formula (101),structural formula (102), structural formula (103), structural formula(105), structural formula (106), structural formula (107), andstructural formula (108)); and10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole (abbreviation: Dic(structural formula (R01))) and PCCP (abbreviation) (structural formula(R02)) which are used as comparative examples were calculated by quantumchemistry calculation.

The most stable structures in the singlet state and in the triplet statewere obtained by calculation using the density functional theory. As abasis function, 6-311G was applied to all the atoms. Furthermore, toimprove calculation accuracy, the p function and the d function aspolarization basis sets were added respectively to hydrogen atoms andatoms other than hydrogen atoms. As a functional, B3PW91 was used.Further, each of the HOMO level and the LUMO level of the structure inthe singlet state was calculated. Gaussian 09 was used as the quantumchemistry computational program.

Table 5 shows the calculation results.

TABLE 5 Structural HOMO level LUMO level T1 level formula No. (eV) (eV)(eV) (100) −5.14 −0.94 2.75 (101) −5.13 −1.58 2.46 (102) −5.13 −1.592.42 (103) −5.08 −0.86 2.76 (104) −5.16 −1.31 2.72 (105) −5.11 −1.362.68 (106) −5.48 −1.40 2.74 (107) −5.62 −1.54 2.70 (108) −5.15 −1.282.75 (R01) −5.29 −0.86 2.90 (R02) −5.30 −1.03 2.72

Table 5 shows that the HOMO level of P3Dic (abbreviation) (structuralformula (100)) is high. This is considered to derive from thediindolocarbazole skeleton of Dic (abbreviation) (structural formula(R01)) having a high HOMO level as well as P3Dic (abbreviation).Further, P3Dic (abbreviation) has a structure in which a phenyl grouphas been bonded to each of the 5-position, the 10-position, and the15-position of Dic (abbreviation), and the diindolocarbazole skeleton isbonded to the substituents at the 5-position, the 10-position, and the15-position, whereby extension of conjugation to these substituents canbe suppressed; therefore, it is considered that P3Dic (abbreviation) canmaintain a high T1 level as well as Dic (abbreviation). Furthermore, itis indicated that, in the case of a structure in which an alkyl group isbonded to the phenyl group of P3Dic (abbreviation) as in a compoundshown by the structural formula 103, the HOMO level can be higher thanthat of P3Dic (abbreviation) while the T1 level is as high as that ofP3Dic (abbreviation).

Further, BP3Dic (abbreviation) shown by the structural formula 104 has astructure in which a para-biphenyl group has been bonded to each of the5-position, the 10-position, and the 15-position of Dic (abbreviation),and has almost the same HOMO level as P3Dic (abbreviation) and hassufficiently high T1 level as well as P3Dic (abbreviation). Furthermore,a compound shown by the structural formula 108 has a structure in whicha meta-biphenyl group has been bonded to each of the 5-position, the10-position, and the 15-position of Dic (abbreviation), and it is foundthat the compound has almost the same HOMO level as P3Dic (abbreviation)and high T1 level as well as P3Dic (abbreviation).

Further, it is found that the HOMO levels of compounds shown by thestructural formula 101, the structural formula 102, and the structuralformula 105 are as high as that of P3Dic (abbreviation), and the LUMOlevel and the T1 level of the P3Dic (abbreviation) are higher than thoseof the compounds. This is considered to be because in the case of thestructure in which a phenyl group has been bonded to each of the5-position, the 10-position, and the 15-position as in P3Dic(abbreviation), extension of conjugation can be suppressed; however, inthe case of a structure in which a substituent that is more likely toallow extension of conjugation than a phenyl group has been bonded toany of the 5-position, the 10-position, and the 15-position as in thecompounds shown by the structural formula 101, the structural formula102, and the structural formula 105, the compound is affected byextension of conjugation; therefore, the LUMO level is low and the T1level is reduced. It is indicated that the driving voltage of alight-emitting element formed using these compounds can be reducedbecause the LUMO level is low.

Further, compounds shown by the structural formula 106 and thestructural formula 107 each have a structure in which pyrimidine whichis a heterocyclic ring is bonded to each of the 5-position, the10-position, and the 15-position instead of the phenyl group of P3Dic(abbreviation). Note that pyrimidine is considered to have a deeper HOMOlevel and a deeper LUMO level than P3Dic (abbreviation) becausepyrimidine is an electron-deficient skeleton. However, it is found thatsince pyrimidine is a six-membered ring, conjugation is less likely toextend; therefore, the T1 level of pyrimidine can be as high as that ofP3Dic (abbreviation). Accordingly, it is found that the compounds shownby the structural formula 106 and the structural formula 107 arecompounds having a bipolar property and a high T1 level. Further, it isfound that the compounds are suitably used as a host material for amaterial emitting phosphorescence having a relatively short wavelength.

Next, table 6 shows values of the HOMO level and the LUMO level obtainedby CV measurement, the peak wavelength of fluorescence spectrum, thepeak wavelength of phosphorescence spectrum, and the T1 level in a thinfilm of each of P3Dic (abbreviation) (structural formula (100)), BP3Dic(abbreviation) (structural formula (104)), and PCCP (abbreviation)(structural formula (R02)).

TABLE 6 CV measurement Structural HOMO LUMO formula level levelfluorescent phosphorescent T1 level No. (eV) (eV) peak (nm) peak (nm)(eV) (100) −5.51 — 397 449 2.76 (104) −5.51 — 413 463 2.68 (R02) −5.63 —411 469 2.64

Also from the measured values, it is found that P3Dic (abbreviation) andBP3Dic (abbreviation) have high HOMO levels. Further, it is found thatP3Dic (abbreviation) and BP3Dic (abbreviation) have high T1 levels andcan be used as a host material for a material emitting light in avisible light region.

Note that the thin films were cooled to 10 K and then irradiated withthe excitation light to obtain emission spectra, which were calculatedby a time-resolved method to find the peaks of phosphorescence. The T1levels shown here are the values obtained by conversion from these peakvalues of phosphorescence to energy values.

REFERENCE NUMERALS

10: light-emitting layer, 11: first organic compound (host material),12: second organic compound (guest material), 13: hole-transport layer,14: host material, 101: anode, 102: cathode, 103: EL layer, 104:light-emitting layer, 105: first organic compound (host material), 106:second organic compound (guest material), 201: first electrode (anode),202: second electrode (cathode), 203: EL layer, 204: hole-injectionlayer, 205: hole-transport layer, 206: light-emitting layer, 207:electron-transport layer, 208: electron-injection layer, 209: firstorganic compound (host material), 210: second organic compound (guestmaterial), 301: first electrode, 302(1) to 302(n): EL layer, 304: secondelectrode, 305: charge-generation layer, 305(1) to 305(n−1): chargegeneration layer, 501: element substrate, 502: pixel portion, 503:driver circuit portion (source line driver circuit), 504 a, 504 b:driver circuit portion (gate line driver circuit), 505: sealant, 506:sealing substrate, 507: wiring, 508: FPC (flexible printed circuit),509: n-channel TFT, 510: p-channel TFT, 511: switching TFT, 512: currentcontrol TFT, 513: first electrode (anode), 514: insulator, 515: ELlayer, 516: second electrode (cathode), 517: light-emitting element,518: space, 1100: substrate, 1101: first electrode, 1102: EL layer,1103: second electrode, 1111: hole-injection layer, 1112: hole-transportlayer, 1113: light-emitting layer, 1114: electron-transport layer, 1115:electron-injection layer, 7100: television device, 7101: housing, 7103:display portion, 7105: stand, 7107: display portion, 7109: operationkey, 7110: remote controller, 7201: main body, 7202: housing, 7203:display portion, 7204: keyboard, 7205: external connection port, 7206:pointing device, 7301: housing, 7302: housing, 7303: joint portion,7304: display portion, 7305: display portion, 7306: speaker portion,7307: recording medium insertion portion, 7308: LED lamp, 7309:operation key, 7310: connection terminal, 7311: sensor, 7312:microphone, 7400: mobile phone, 7401: housing, 7402: display portion,7403: operation button, 7404: external connection port, 7405: speaker,7406: microphone, 8001: lighting device, 8002: lighting device, 8003:lighting device, 8004: lighting device, 9033: clip, 9034: switch forswitching display modes, 9035: power button, 9036: switch for switchingto power-saving mode, 9038: operation button, 9630: housing, 9631:display portion, 9631 a: display portion, 9631 b: display portion, 9632a: touch panel area, 9632 b: touch panel area, 9633: solar battery,9634: charge and discharge control circuit, 9635: battery, 9636: DCDCconverter, 9637: operation key, 9638: converter, and 9639: button.

This application is based on Japanese Patent Application serial no.2012-208080 filed with Japan Patent Office on Sep. 21, 2012, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A light-emitting device comprising: a pairof electrodes; and a layer between the pair of electrodes, the layercomprising a compound represented by a formula (G1) and a phosphorescentcompound,

wherein: a difference between a HOMO level of the compound and a HOMOlevel of the phosphorescent compound is lower than or equal to 0.3 eV,α¹ to α³ independently represent a substituted or unsubstitutedphenylene group or a substituted or unsubstituted biphenyldiyl group,Ar¹ to Ar³ independently represent any of a substituted or unsubstitutedphenyl group, a substituted or unsubstituted pyridyl group, asubstituted or unsubstituted pyrimidyl group, a substituted orunsubstituted naphthyl group, a substituted or unsubstituted fluorenylgroup, a substituted or unsubstituted triphenylenyl group, and asubstituted or unsubstituted phenanthrenyl group, and l, m and n areindependently 0 or
 1. 2. The light-emitting device according to claim 1,wherein the compound is represented by any one of the structuralformulae (100), (104), and (108)


3. The light-emitting device according to claim 1, wherein thephosphorescent compound is an organometallic complex.
 4. Thelight-emitting device according to claim 1, wherein the phosphorescentcompound comprises iridium.
 5. An electronic device comprising thelight-emitting device according to claim
 1. 6. A lighting devicecomprising the light-emitting device according to claim
 1. 7. Alight-emitting device comprising: a pair of electrodes; a first layerbetween the pair of electrodes, the first layer comprising a firstcompound represented by a formula (G1); and a second layer in contactwith the first layer, the second layer comprising a second compoundrepresented by the formula (G1) and a phosphorescent compound,

wherein: a difference between a HOMO level of the second compound and aHOMO level of the phosphorescent compound is lower than or equal to 0.3eV, α¹ to α³ independently represent a substituted or unsubstitutedphenylene group or a substituted or unsubstituted biphenyldiyl group,Ar¹ to Ar³ independently represent any of a substituted or unsubstitutedphenyl group, a substituted or unsubstituted pyridyl group, asubstituted or unsubstituted pyrimidyl group, a substituted orunsubstituted naphthyl group, a substituted or unsubstituted fluorenylgroup, a substituted or unsubstituted triphenylenyl group, and asubstituted or unsubstituted phenanthrenyl group, and l, m and n areindependently 0 or
 1. 8. The light-emitting device according to claim 7,wherein the first compound is different from the second compound.
 9. Thelight-emitting device according to claim 7, wherein at least one of thefirst compound and the second compound is represented by any one of thestructural formulae (100), (104), and (108)


10. The light-emitting device according to claim 7, wherein thephosphorescent compound is an organometallic complex.
 11. Thelight-emitting device according to claim 7, wherein the phosphorescentcompound comprises iridium.
 12. An electronic device comprising thelight-emitting device according to claim
 7. 13. A lighting devicecomprising the light-emitting device according to claim 7.